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Why does the presence of tension reinforcement lead to increasing deflection in concrete structures?

In BS8110 a modification factor is applied to span/depth ratio to take into account the effect of tension reinforcement. In fact, deflection of concrete structure is affected by the stress and the amount of tension reinforcement.

To illustrate their relationship, let’s consider the following equation relating to beam curvature:

Curvature = 1/r = e/(d-x)
where r = radius of curvature
e = tensile strain in tension reinforcement
d = effective depth
x = depth of neutral axis

Provided that the tensile strain in tension reinforcement remains constant, the curvature of concrete structure increases with the depth of neutral axis. It is observed that the depth of neutral axis rises with tension steel ratio. Therefore, the curvature of concrete section is directly proportional to the tension steel ratio. In addition, the larger value of the depth of neutral axis enhances increased area of concrete compression so that the effect of creep on deflection appears to become significant.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What is the purpose of setting minimum amount of longitudinal steel areas for columns?

In some design codes it specifies that the area of longitudinal steel reinforcement should be not less than a certain percentage of the sectional area of column. Firstly, the limitation of steel ratio for columns helps to guard against potential failure in tension. Tension may be induced in columns during the design life of the concrete structures. For instance, tension is induced in columns in case there is uneven settlement of the building foundation, or upper floors above the column are totally unloaded while the floors below the column are severely loaded. Secondly, owing to the effect of creep and shrinkage, there will be a redistribution of loads between concrete and steel reinforcement. Consequently, the steel reinforcement may yield easily in case a lower limit of steel area is not established.

In addition, test results showed that columns with too low a steel ratio
would render the equation below inapplicable which is used for the design of columns:

N=0.67fcuAc+fyAs

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Why are longer tension lap lengths designed at the corners and at the top of concrete structures?

In BS8110 for reinforced concrete design, it states that longer tension lap lengths have to be provided at the top of concrete members. The reason behind this is that the amount of compaction of the top of concrete members during concrete placing is more likely to be less than the remaining concrete sections. Moreover, owing to the possible effect of segregation and bleeding, the upper layer of concrete section tends to be of lower strength when compared with other locations.

When the lap lengths are located at the corners of concrete members, the degree of confinement to the bars is considered to be less than that in other locations of concrete members. As such, by taking into account the smaller confinement which lead to lower bond strength, a factor of 1.4 (i.e. 40% longer) is applied to the calculated lap length.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Does longitudinal steel serve as an enhancement of shear strength?

In addition to shear resistance provided by shear reinforcement, shear forces in a concrete section is also resisted by concrete compression force (compressive forces enhances higher shear strength), dowel actions and aggregate interlocking. The presence of longitudinal steel contributes to the enhancement of shear strength of concrete section in the following ways:

(i) The dowelling action performed by longitudinal reinforcement directly contributes significantly to the shear capacity.

(ii) The provision of longitudinal reinforcement also indirectly controls the crack widths of concrete section which consequently affects the degree of interlock between aggregates.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Why does lap length generally greater than anchorage length?

In some structural codes, the lap length of reinforcement is simplified to be a certain percentage (e.g. 25%) higher than the anchorage length. This requirement is to cater for stress concentrations at the end of lap bars. A smaller load when compared with the load to pull out an anchored bar in concrete triggers the splitting of concrete along the bar because of the effect of stress concentration. A higher value of lap length is adopted in design code to provide for this phenomenon.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.